1. Field of the Invention
The present invention relates to a recording head of an electrostatic attraction type image recording apparatus used in printers, facsimiles, and the like. More particularly, the present invention relates to a recording head of an electrostatic attraction type image recording apparatus that can be manufactured at low cost.
2. Description of the Background Art
Various image recording devices are known which use conventional recording head such as an ink jet printer. One such device uses a pressure chamber wall that mechanically deforms to reduce the volume of the pressure chamber and eject ink for printing on a print medium. Also, a bubble jet type image recording apparatus is well known that utilizes vaporization of ink heated instantaneously with a heater to increase pressure in the pressure chamber to transfer ink.
A system for printing on a print medium, not by crating by pressure change to transfer ink, but by drawing ink by electrostatic attraction induced by introducing charge into conductive ink is defined in Japanese Patent Publication No. 36-13768. According to this system, ink is not sprayed and instead is attracted onto a print medium (recording medium) for printing (recording) by an electrostatic force exerted on the ink by applying a voltage between a record (control) electrode and an opposite electrode.
An improvement of the above electrostatic attraction type recording apparatus is disclosed in Japanese Patent Laying-Open No. 7-223317. This apparatus has a needle-like control electrode provided at the ink outlet as a record (control) electrode.
Although the diameter of an ink droplet from an ink jet printer that has ink transferred by a pressure change in the pressure chamber depends upon physical properties such as the surface tension of ink and the like, it is greater than the diameter of the orifice from which ink is ejected.
Since the transferred ink spreads 2-3 times the sprayed-out ink diameter on a recording medium such as a paper sheet, the diameter of the ink dot on the recording medium becomes considerably greater than the diameter of the orifice.
In a printer that uses a piezoelectric element as a pressure generation source, the head size cannot be reduced since it is difficult to miniaturize the piezoelectric element. The nozzle could not be provided at high integration, thus causing the problem that the print out speed is low.
In a bubble jet type printer, the temperature of the ink is altered suddenly and repeatedly. This has caused a problem that the ink quality is degraded and that the life time of the heater is not sufficient.
In an electrostatic attraction type image recording apparatus, the charge injected into conductive ink is attracted towards the opposite electrode by electrostatic force. The ink is pulled in a string-like manner to arrive at the printing face of a recording medium in a particle form or still in the string-like form. The droplet or string of ink can be adjusted to be as small as approximately 10 .mu.m in diameter. Thereby enabling the high print quality.
Furthermore, such an apparatus is basically implemented by electrodes for generating an electric field between the orifice and the recording medium. Therefore, the structure thereof is extremely simple.
However, the ink cannot be stably provided in a string-like manner just by applying voltage between the electrodes. When voltage is applied across the electrodes, charge is injected into the conductive ink, whereby the charge is concentrated at the leading end of the ink. Since the surface of the ink is concave within the nozzle due to surface tension, the charge will be concentrated on the perimeter of the orifice. The ink could be drawn out from anywhere on the circumference. There is a possibility that a plurality of strings of ink will be generated from one orifice, thereby significantly the quality of the printout.
In view of the foregoing, there are several electrostatic attraction systems as set forth in the following that has a convex meniscus formed to stably generate string-like ink.
One system forms a convex meniscus by setting the ink tank higher than the orifice, or by constantly applying static pressure from the backside of the pressure chamber. Another system forms a convex meniscus by generating a traveling wave in the ink or by applying pressure periodically to the pressure chamber by an actuator such as a piezoelectric element. Voltage is applied in synchronization thereof to generate ink in a string-like manner.
The former system is disadvantageous in that ink will leak out from the orifice when the balance between the static pressure and the ink surface tension is destroyed, since a convex meniscus is constantly formed. There is also a disadvantage that, when a foreign object such as a paper particle is attached at the neighborhood of the orifice, ink will leak out through the foreign object.
Also, there is a limit in recording at high frequency since there is some time period before the initial meniscus status is restored. The ink consumed by being transferred out is supplied again by hydrostatic pressure, whereby the initial meniscus status is restored. Furthermore, since the electrodes are provided in the neighborhood of the meniscus, there is a problem that charge injection is too slow in effecting high frequency drive.
The problem of ink leakage is not so serious in the latter system. However, the latter system further requires a device to form a convex meniscus in addition to the electrodes, and lacks the advantage of a simple structure of the electrostatic attraction system. Thus, such a design is inferior in regard cost and size of the image recording apparatus.
Japanese Patent Laying-Open No. 7-223317 discloses an image recording apparatus having a structure in which a needle-type member is provided at the ink outlet. Ink travels along the needle-type member by means of surface tension to promote refill of the meniscus. The needle-like member can also be used as an electrode to reduce the time required for introducing charge.
The disadvantage of the image recording apparatus described in Japanese Patent Laying-Open No. 7-223317 is as follows. According to the disclosed embodiment, the needle-like member is formed by providing a plurality of layers of a metal material by means of plating. The needle-like member has a diameter of 20 .mu.m and a length which is the sum of the portion protruding 30 .mu.m from the orifice, the thickness of the overhead plate, and the portion in the cavity. In a general structure, the length of the needle-like member is as long as 100 .mu.m and greater.
In order to provide plating of a small pattern such as several ten .mu.m in diameter, the portion which is not to be plated is covered with a resist film and the like by photolithography. Then, plating is carried out.
However, it is extremely difficult to achieve accurately a resist pattern of a high aspect ratio having 20 .mu.m in diameter and at least 100 .mu.m in height.
At the final stage of connecting the needle-like member and the overhead plate, there is a problem that the extremely fine needle-type member is damaged, thereby degrading the yield of the head and increasing. Furthermore, since the positioning of the connection cannot be carried out precisely, the needle-like member may deviate from the center and result in an asymmetrical meniscus configuration. This causes the problem that the direction of the spraying ink is not stable, thereby is a possibly degarding the print quality.